iPhone 4S (iOS 5.1) alarm issue

I think I was having this problem before, but eventually some combination of updating and troubleshooting seemed to have fixed it. As soon as I updated to iOS 5.1, however, it came back.

Here's the problem: say I set alarms for 7:00, 7:15, and 7:30. When 7:00 rolls around, the first alarm goes off without any problems. However, if I don't wake up in time to turn off this first alarm, when the clock hits 7:15, although the alarm band shows on the phone's screen as if it is going off, the phone doesn't make any noise, vibrate or anything. Once the clock hits 7:30, again, the alarm is technically going off, but the phone isn't making any noise.

Anyway, I've tried troubleshooting and it did nothing. Any ideas what the problem is or suggestions?